Marek Malac

Fabrication of submicrometer regular arrays of pillars and helices

We use a glancing-angle deposition technique to produce regular lattices of submicrometer pillars and helices with a two-dimensional lattice constant below 1 μm and structure heights of 0.5–10 μm. Possible applications of such structures include photonic crystals and magnetic-storage media.

Observations of the microscopic growth mechanism of pillars and helices formed by glancing-angle thin-film deposition

We have studied the mechanisms influencing growth of thin films onto an oblique rotating substrate by cross-sectional transmission electron microscopy and scanning electron microscopy. We have analyzed the growth of pillars and helices in random and regular arrays, and have examined the influence of introducing a line of missing nuclei on the growth of regular array of pillars and helices.

Periodic magnetic microstructures by glancing angle deposition

An advanced deposition technique known as glancing angle deposition (GLAD) [K. Robbie, J. C. Sit, and M. J. Brett, J. Vac. Sci. Technol. B 16, 1115 (1998); K. Robbie and M. J. Brett, U.S. Patent No. 5,866,204 (filed 1999)] has been used to fabricate periodic arrays of magnetic pillars and randomly seeded magnetic helices, posts, and chevrons. Because of the nature of initial film nucleation, the GLAD process normally distributes posts randomly on the substrate surface. We can grow periodic arrays of GLAD microstructures by suppressing the randomness inherent within the initial nucleation stage of film growth. Shadowing sites were fabricated by pre-patterning a thin titanium layer on silicon substrates into a square array using electron beam lithography. These sites shadow regions of the substrate from incident flux during film deposition and act as preferred nucleation sites for film growth. Using this process, we have fabricated periodic arrays of cobalt posts with a regular elemental period of 600 nm an...

Heteroepitaxial Growth of Gold Nanostructures on Silicon by Galvanic Displacement

This work focuses on the synthesis and interfacial characterization of gold nanostructures on silicon surfaces, including Si(111), Si(100), and Si nanowires. The synthetic approach uses galvanic displacement, a type of electroless deposition that takes place in an efficient manner under aqueous, room-temperature conditions. The case of gold-on-silicon has been widely studied and used for several applications and yet, a number of important, fundamental questions remain as to the nature of the interface. Some studies are suggestive of heteroepitaxial growth of gold on the silicon surface, whereas others point to the existence of a silicon-gold intermetallic sandwiched between the metallic gold and the underlying silicon substrate. Through detailed high resolution transmission electron microscopy (TEM), combined with selected area electron diffraction (SAED) and nanobeam diffraction (NBD), heteroepitaxial gold that is grown by galvanic displacement is confirmed on both Si(100) and Si(111), as well as silicon nanowires. The coincident site lattice (CSL) of gold-on-silicon results in a very small 0.2% lattice mismatch due to the coincidence of four gold lattices to three of silicon. The presence of gold-silicon intermetallics is suggested by the appearance of additional spots in the electron diffraction data. The gold-silicon interfaces appear heterogeneous with distinct areas of heteroepitaxial gold on silicon, and others, less well-defined, where intermetallics may reside. The high resolution cross-sectional TEM images reveal a roughened silicon interface under these aqueous galvanic displacement conditions, which most likely promotes nucleation of metallic gold islands that merge over time: a Volmer-Weber growth mechanism in the initial stages.

Convenient contrast enhancement by a hole-free phase plate

Decrease of the irradiation dose needed to obtain a desired signal-to-noise ratio can be achieved by Zernike phase-plate imaging. Here we present results on a hole-free phase plate (HFPP) design that uses the incident electron beam to define the center of the plate, thereby eliminating the need for high precision alignment and with advantages in terms of ease of fabrication. The Zernike-like phase shift is provided by a charge distribution induced by the primary beam, rather than by a hole in the film. Compared to bright-field Fresnel-mode imaging, the hole-free phase plate (HFPP) results in two- to four-fold increase in contrast, leading to a corresponding decrease in the irradiation dose required to obtain a desired signal-to-noise ratio. A local potential distribution, developed due to electron beam-induced secondary-electron emission, is the most likely mechanism responsible for the contrast-transfer properties of the HFPP.

Water-Soluble J-Type Rosette Nanotubes with Giant Molar Ellipticity

A new self-assembling tricyclic module (×K1) featuring the Watson-Crick H-bonding arrays of guanine and cytosine fused to an internal pyridine ring was synthesized. When dissolved in water at room temperature, this module rapidly self-assembles into hexameric rosettes, which then stack to form J-type rosette nanotubes (RNTs) with increased inner/outer diameters and the largest molar ellipticity ever reported (4 × 10(6) deg·M(-1)·m(-1)). Using a combination of imaging and spectroscopic techniques we established the structure of ×K1-RNT and have shown that the extended π system of the self-assembling module resulted in a new family of J-type RNTs with enhanced intermodular electronic communication.

Improved background-fitting algorithms for ionization edges in electron energy-loss spectra

We discuss improved procedures for fitting a power-law background to an ionization edge in an electron energy-loss spectrum. They place constraints on the background, both above and below the ionization-threshold energy, and are of particular advantage in the case of weak edges arising from low elemental concentrations. The algorithms are currently implemented as short Calculator programs for Gatan EL/P software. Their advantages and limitations are discussed, in comparison with multiple-least-squares and spatial-difference techniques.

Local thickness measurement through scattering contrast and electron energy-loss spectroscopy

Scattering contrast measurements were performed on thin films of amorphous carbon and polycrystalline Au, as well as single-crystal MgO nanocubes. Based on the exponential absorption law, mass-thickness can be obtained within 10% accuracy by measuring the incident and transmitted intensities in the same image. For mass-thickness measurement of a thin amorphous specimen, a small collection semiangle improves the measurement sensitivity, whereas for the measurement of polycrystalline or single-crystal specimens, a large collection semiangle should be used to reduce diffraction-contrast effects. EELS thickness measurements on MgO nanocubes suggest that the Kramers-Kronig sum-rule method (with correction for plural and surface scattering) gives 10% accuracy at medium collection semiangles but overestimates the thickness at small collection semiangles, due to underestimation of the surface-mode scattering. The log-ratio method, with a formula for inelastic mean free path proposed by Malis et al. (1988), provides 10% accuracy at small collection semiangle, while that proposed by Iakoubovskii et al. (2008a) is preferable for medium and large collection semiangles. As a result of this work, we provide recommendations of preferred methods and conditions for local-thickness measurement in the TEM.

Thin-film regular-array structures with 10-100 nm repeat distance

We show that regular arrays of pillars and helices, with repeat distance below 100 nm, can be grown by oblique deposition onto a patterned rotating substrate. The size limits for such structures are discussed.

Fourier-ratio deconvolution techniques for electron energy-loss spectroscopy (EELS).

We discuss several ways of using Fourier-ratio deconvolution to process low-loss spectra. They include removal of the tail arising from the zero-loss peak, extraction of the spectrum of a particle from data recorded from the particle on a substrate, separation of the bulk and surface components in spectra recorded from samples of the same composition but different thickness, and investigation of interface energy-loss modes. We also demonstrate the use of a Bayesian-equivalent procedure based on the Richardson-Lucy algorithm.